The aspects of the present disclosure relate generally to the field of occupancy sensors, and in particular to long range occupancy sensors.
Due to a desire to save energy as well as new building codes, automatic lighting control devices are increasing in popularity. One common type of automatic lighting control is the single-room occupancy or vacancy sensor. These devices use one or more sensing means to determine whether a room is occupied or vacant and control the lighting for the room accordingly. Occupancy sensors can be positioned in a room in several ways. One of the most common arrangements is to replace a standard wall switch in a wall box with an occupancy sensor. This arrangement, generally referred to as a wallbox sensor, gives the occupancy sensor access to the lighting circuit both for switching and for powering its internal circuitry. Due to historical wiring techniques, however, a neutral AC connection is rarely available in this arrangement. These devices draw operational current by utilizing ground leakage currents. Therefore, the ground connection must be utilized for current return. This wiring environment is often referred to as a two-wire connection. When a neutral AC connection is available along with the AC hot and ground connection, it will be referred to as a three-wire connection. The terms “hot”, “neutral” and “ground” connections are generally intended to encompass their ordinary and understood meanings where “hot’ is any conductor connected within an electrical system that has electrical potential relative to electrical ground or neutral; “neutral” is the return leg of an electrical power circuit; and “ground” is a conductor with continuity to earth ground potential. An electrical outlet is a point in the wiring system where current is delivered to equipment utilizing electrical power.
The sensing technologies can be broadly classified as passive technologies, which only receive energy from the room, or active technologies, which transmit energy into the room. An active sensor, such as an ultrasonic or microwave sensor, uses a transmitter to emit energy into a space to be monitored. The greater the energy transmitted, the larger the space that can be monitored. The sensor then uses a receiver to collect the reflected energy and analyze it for signs of occupancy within the space. The receiver portion is low-power since no energy must be emitted. Active sensors or devices have the transmitter and receiver co-located in the same device.
As noted above, it is common for occupancy sensors to be installed as replacements for standard wall switches, giving the sensor access to the AC lighting circuit for switching and powering of its internal circuitry. However, since AC neutral wiring is not provided in these arrangements, the power used by the sensor must be limited, and the ground connection is utilized for current return. However, the amount of ground return leakage current is typically limited by safety standards, to 500 microamperes for example. Active sensing technologies struggle to adhere to this limitation and the power availability can be inadequate to monitor a large space. If an alternative to ground leakage current, such as battery power or photovoltaic power, is used to power the device, power availability is similarly limited.
Because these technologies exhibit good performance when detecting minor motion, they have become an increasingly larger part of modern sensors. The more power that is available to active sensors, the better their operational sensing range. Therefore, occupancy sensors subject to the power restrictions listed above will be subject to range and performance limitations if they employ active sensing. It would be advantageous to provide an active sensor arrangement which avoids the power limitations of no-neutral installations for greater sensing range.
Ultrasonic sensors operate by detecting a Doppler frequency shift between the transmitted and received signals. Doppler frequency shifts are generated by the ultrasonic energy that is reflected off of a moving object in the space. The magnitude of the frequency shift is proportional to speed of the moving object. In one embodiment, Doppler shifts greater than approximately 40 Hz are interpreted as motion with in the space. Thus, the receiver unit must have access to the original transmitted waveform in order to reliably determine the occupancy status of a room. Active sensors will have the transmitter and the receiver co-located in the same package to enable such access to the transmitted waveform. However, such co-location will limit the power that is available to the transmitter from the leakage ground current. It would be advantageous to be able to enable the receiver to accurately detect occupancy in the space without direct access to the original transmitted waveform.
Accordingly, it would be desirable to provide an occupancy sensor that addresses at least some of the problems identified above.